1. Field of the Invention
The present invention relates to a small multiple-resonance antenna, more specifically a dipole antenna and an inverted-L antenna that includes a plurality of parallel metal wires as its basic structure and a plurality of identical or similar unit circuits arranged in a row in the direction of the metal wires and connected with each other.
2. Description of the Related Art
As stated in R. A. Shelby, D. R. Smith, and S. Schults, “Experimental verification of a negative index of refraction”, Science, vol. 292, pp. 77-79, April 2001, left-handed materials have been studied and applied to antennas vigorously since their appearance. The term “left-handed material” refers to a material having both a negative permittivity and a negative magnetic permeability. When an electromagnetic wave is propagated in the left-handed material, its group velocity and phase velocity of the electromagnetic wave are opposite in direction. The left-handed material also shortens the wavelength of the electromagnetic wave as its frequency decreases.
Examples of antennas that operate as a left-handed antenna include: a leaky wave antenna disclosed in L. Liu, C. Caloz, and T. Ito, “Dominant mode Leaky-wave antenna with backfire-to-endfire scanning capability”, Electron. Lett., vol. 38, no. 23, pp. 1414-1416, November 2002; a small antenna formed on a ground disclosed in M. Schuessler, J. Freese, and R. Jakoby, “Design of compact planar anarantennas using LH-transmission lines”, 2004 IEEE MTT-S Int. Microwave Symp. Dig., vol. 1, pp. 209-212, Fort Worth, Tex., June 2004, C. J. Lee, K. M. H. Leong, and T. Itoh, “Design of resonant small antenna using composite right/left-handed transmission line”, IEEE Int. Antennas Propagat. Symp. Dig., vol. 2B, pp. 218-221, Washington D.C., July 2005, and F. Qureshi, M. A. Antoniades, and G. V. Eleftheriades, “A compact and low-profile metamaterial ring antenna with vertical polarization”, IEEE Antennas and Wireless Propagat. Lett., vol. 4, pp. 333-336, 2005; and a dipole antenna disclosed in Japanese Patent Application Publication No. 2006-295873 (JP-A-2006-295873).
FIG. 21 shows a straight dipole antenna A10 that operates as a left-handed antenna disclosed in JP-A-2006-295873. The dipole antenna A10 includes two parallel metal wires p10, q10 as its basic structure, and six unit circuits U10 having length a and connected in the x-axis direction (the direction of the metal wires p10, q10). The unit circuits U10 are each composed of two capacitors CSE10 connected in series on a part of the metal wire p10, and a tie portion that ties the metal wires p10 and q10 via an inductor LSH10. A feed point F constituted of two points FL, FR is positioned at the middle of the metal wire p10.
With the capacitors CSE10 and the inductors LSH10 arranged periodically, the dipole antenna A10 may operate as a left-handed antenna. As described above, the left-handed material reduces the wavelength shorter as the frequency decreases. Therefore, the antenna length L10 of the dipole antenna A10 may be reduced to about one tenth the operating wavelength by controlling the capacitance of the capacitor CSE10 and the inductance of the inductor LSH10.
A dual-resonance antenna that operates with a right-handed material and a left-handed material is disclosed in S. Otto, A. Rennings, C. Caloz, P. Waldow, and T. Itoh, “Composite Right/Left-Handed λ-Resonator Ring Antenna for Dual-Frequency Operation”, IEEE Int. Antennas Propagat. Symp. Dig., vol. 1A, pp. 684-687, Washington D.C., July 2005.
The frequency band of electromagnetic waves allocated to tire air pressure warning systems and smart entry systems, which are in-vehicle applications, is the 400 MHz band in Europe, and the 300 MHz band in North America and Japan. Antennas for use used in these systems are preferably small, because their installation space is occasionally limited, and can able to use both the two frequency bands, namely the 300 MHz band and the 400 MHz band, because their installation space is occasionally limited.
In normal right-handed antennas, if a first resonance occurs at 300 MHz, for example, a second resonance occurs at about 900 MHz, about three times the frequency of the first resonance. The dipole length is equivalent to half the operating wavelength for the first resonance, and 1.5 times the operating wavelength for the second resonance. Since there is a wide gap between the frequencies of the first and second resonances as described above, right-handed antennas cannot be used for the applications mentioned above.
In contrast, in left-handed antennas, decreases in frequency shorten the wavelength and reduce the gap between the frequencies. That is, a first resonance may occur at about 400 MHz (half the wavelength), and a second resonance may occur at about 300 MHz (1.5 times the wavelength). When the antenna length is short relative to the operating wavelength, however, the dipole antenna A10 in accordance with the above related art does not operate as an antenna, because currents flow in opposite directions to cancel each other at the second resonance.
With the dual-resonance antenna that operates as a left-handed antenna and a right-handed antenna disclosed in IEEE Int. Antennas Propagat. Symp. Dig., vol. 1A, pp. 684-687, Washington D.C., July 2005, the gap between the resonance frequencies cannot be reduced. In addition, it is necessary to improve the radiation efficiency by impedance matching at the feed point.